In high speed (HS) systems, the downlink (DL) carrier power varies very much TTI by TTI due to the nature of packet burst transmission, causing much fluctuation in the intra-cell interference. Transmission Time Interval (TTI) is defined as the inter-arrival time of a Transport Block Set (TBS), and is equal to the periodicity at which a Transport Block Set is transferred by the physical layer on the radio interface. It is always a multiple of the minimum interleaving period. The Medium Access Control (MAC) delivers one Transport Block Set to the physical layer every TTI.
The Channel Quality Indicator (CQI) is a quantized indicator of the channel quality experienced by the user equipment, and may for example comprise a carrier/interference ratio (C/I) quantized into a predetermined range of dB values. The CQI is generally broadcast at a high rate, e.g., 800 Hz, or once every 1.25 msec. The CQI thus provides a high rate channel feedback mechanism to report the relative C/I at each user equipment. The reported Channel Quality Indicator (CQI) from a user equipment (UE) to the corresponding base station reflects the channel quality only for the previous TTI, not the channel quality that the data will experience in current TTI.
The main problem with CQI reports not mirroring actual channel conditions will be that the radio access network (RAN), such as the UMTS Terrestrial RAN (UTRAN), performs the link adaptation based on faulty data leading to an inefficient use of the high speed-downlink shared channel (HS-DSCH). This will manifest either by excessive retransmissions in case the UE overestimates the channel or by lower throughput and wasted capacity in case the UE underestimates the current channel conditions.
Deviating CQI reports may also lead to faulty scheduling decisions since the channel condition is an input to the scheduling selection algorithm in the cases where proportional fair (PF) or max carrier-to-interference (C/I) scheduling schemes are employed. Thus a UE that consistently overestimates the channel will be rewarded by more frequent scheduling.
Existing solutions to the above problem focus on the CQI adjustment to improve its accuracy, which are mainly based on three input parameters, namely, the downlink transmission power control (DL TPC), MAC-hs acknowledgement/negative acknowledgement (ACK/NACK) rate and Doppler estimation.
Obviously, the existing solutions put efforts to improve the received CQI accuracy. However, the fact is that even without any CQI generation error at UEs or CQI transmission error in the uplink (UL), the perfect received CQI reflects the channel quality still only for previous TTI. In addition, using MAC-hs ACK/NACK rate based CQI adjustment, which is a typical solution, could cause one risk that if no HS data transmission and therefore no ACK/NACK reporting, the CQI adjustment can't be performed.